1. Field of the Invention
The present invention relates generally to fiber Bragg gratings, and particularly to suppression of cladding modes in optical fibers which include fiber Bragg gratings.
2. Technical Background
Fiber Bragg gratings are periodic refractive index modulations along the length of an optical waveguiding fiber. Fiber Bragg gratings have become increasingly more important in wavelength division multiplexing (WDM) systems and other applications for fiber optic systems. They have become a technology platform for implementation of a variety of devices including add/drop filters, gain flattening filters, band splitters and dispersion compensators.
Fiber Bragg gratings generally exhibit highly desirable optical characteristics, while being easily fabricated. The most common technique for fabrication of a fiber Bragg grating is to create the pattern of refractive index modulations by exposing the core to ultraviolet light in the desired pattern. The pattern may be created by interference of two ultraviolet light (UV) beams or other well-known means. The refractive index of the core is permanently altered by exposure to the ultraviolet light through the well-known photosensitive effect. This technique is commonly referred to as xe2x80x9cwritingxe2x80x9d a grating Index modulation, which is measured as half of the peak-to-peak variation in refractive index created in the writing process, is a key characteristic of a fiber grating. Index modulation is directly related to photosensitivity of the material in which the grating is written.
A fiber Bragg grating will reflect light in a narrow band centered on the Bragg wavelength, xcexBragg, determined by the equation for the phase matching condition,                               2          ⁢                                    β              01                        ⁡                          (                              λ                Bragg                            )                                      =                              2            ⁢            π                    Λ                                    (        1        )            
where xcex9 is the period of the grating, and xcex201 is the propagation coefficient for the fundamental mode LP01, sometimes also referred to as the core mode.
In an optical waveguiding fiber including a core, a cladding surrounding the core and an outer layer which can be air or a polymer coating, the fiber structure may support a large number of cladding modes. They may be guided modes or leaky modes, depending on whether the outer layer or fiber coating has a lower or a higher refractive index than that of the cladding. These modes are commonly designated as LPnm cladding modes, where nm is the mode number. At a fiber Bragg grating, light propagating in the guided fundamental mode LP01 may couple into a cladding mode under a phase matching condition given by the following equation:                                                         β              01                        ⁡                          (                              λ                nm                            )                                +                                    β              nm                        ⁡                          (                              λ                nm                            )                                      =                              2            ⁢            π                    Λ                                    (        2        )            
where xcex9 is the grating""s period, xcex2nm is the propagation constant of cladding mode LPnm at wavelength xcexnm., and xcex201 is the propagation constant of the fundamental mode LP01 at wavelength xcexnm. The wavelength xcexnm at which LP01 will couple into a cladding mode if equation (2) is fulfilled, is always less than the Bragg wavelength, xcexBragg, because xcex2nm is always less than xcex201.
Typically, a series of wavelengths will meet this condition, corresponding to a series of cladding modes. Power coupled into the cladding modes is typically lost through absorption or scattering through the fiber coating as the cladding modes propagate. Thus, as depicted in FIG. 1, coupling into cladding modes causes a series of loss peaks (designated generally by reference numeral 12) on the short wavelength side of the Bragg wavelength loss peak 14, limiting the free spectral range of the grating. As shown in FIG. 1, there is a wavelength band Axe2x80x94A between the Bragg wavelength peak 14 and the onset of the cladding mode peaks 12. Widening of this band Axe2x80x94A through cladding mode suppression would be desirable to increase the free spectral range of the grating.
One approach to achieving cladding mode suppression has been to use a fiber with a high delta of approximately 2% (where delta is the normalized refractive index difference between the core and cladding). While this provides a free spectral range on the short wavelength side of xcexBragg of as much as about 10 nm, this is still not sufficient for many applications. Another problem with this approach is unacceptably large splice loss when connecting such high delta fiber to standard fiber, such as that sold under the trademark SMF-28(trademark) by Corning Incorporated, due to modal spot size mismatch.
What is needed is an optical waveguiding fiber which has properties which will suppress coupling into cladding modes in fiber Bragg gratings, so as to increase the free spectral range of filters which are made with fiber Bragg gratings, while not adversely affecting other optical properties of the fiber, or the grating.
One aspect of the present invention is an optical waveguiding fiber that has a photosensitive core and a cladding that includes a photosensitive inner cladding region adjacent the core and an outer cladding region. The inner cladding region and the outer cladding region have substantially equal indices of refraction. The core and inner cladding region are doped with Ge. At least one of the core and the inner cladding region is also doped with at least one additional dopant. The concentration of Ge in the core, Ge in the cladding, and the additional dopant are such that the index modulation in the inner cladding region is within 50 percent of the index modulation in the core caused by exposure to actinic radiation such as ultraviolet light.
In another aspect, the present invention includes an optical fiber with a photosensitive core and a photosensitive inner cladding region adjacent the core and an outer cladding region with substantially equal indices of refraction, where the photosensitivity of the inner cladding region is sufficient to cause a modulation of the index of refraction of the inner cladding when exposed to ultraviolet light.
In another aspect of the invention, the optical fiber includes a grating in the core, which extends radially into the inner cladding region.
In another aspect of the invention, the core and the inner cladding region of the optical fiber are doped with concentrations of said Ge and B dopants and are sufficient to impart photosensitivity to the inner cladding region, and to result in an index of refraction in the inner cladding region substantially equal to the index of refraction of the outer cladding region, or within a range of from xe2x88x920.003 to 0.001 for index of refraction of inner cladding region minus that of the remainder of the cladding.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.